Flash memories are based on the technology of either erasable programmable read only memories (EPROMs) or electrically erasable programmable read only memories (EEPROMs). Like EPROMs and EEPROMs, flash memories feature non-volatile data storage and fast read access.
EPROMs are characterized by arrays of single transistor cells. Although individual EPROM cells may be addressed during read and write operations, all cells in an array must be erased simultaneously through exposure to intense UV light. Typically, EPROMs must be removed from a computer system during reprogramming.
EEPROMs, on the other hand, are characterized by two transistor cells. The additional cell, provides individualized access during read, write and erase cycles. This additional flexibility, however, is offset by increased manufacturing costs and reduced device density.
In price and functionality, flash memories fall somewhere between EPROMs and EEPROMs. The density of flash memory rivals that of EPROMs. On the other hand, like EEPROMs, they can be reprogrammed without removal from their resident system.
Those flash devices that are based on EPROM technology are the least expensive and are the most likely candidates for aggressive downsizing and rapid price reductions. However, such devices require a 12-volt power supply for erase and write operations, as well as a 5-volt supply for normal read operations. This kind of device also requires a multi-step algorithm that verifies erasure. Its manufacturers insist that the 12-volt supply helps protect the circuit against accidental erasure. In addition, they note that a 12-volt supply is readily available in some systems such as desktop computers and laser printers. Intel Corporation, the most prominent manufacturer of these devices, and the company most responsible for achieving manufacturability of the device has bestowed upon this type of cell the trademarked name ETOX (for EPROM Tunnel OXide).
Those flash devices, on the other hand, that are based on EEPROM technology, are slightly more expensive. Nevertheless, they provide more flexible reprogramming and require only a 5-volt power supply for read, write and erase functions. The manufacturers of such devices (Texas Instruments, Inc. among them) counter the alleged twin power supply advantage relating to erasure protection with the argument that there are software techniques that may be employed to render accidental erasures too rare to be worth consideration.
The following technical references are incorporated herein as though set forth in full:
Silicon Processing for the VLSI Era, Volume 1--Process Technology, Stanley Wolf and Richard N. Tauber, Lattice Press, 1986;
Silicon Processing for the VLSI Era, Volume 2--Process Technology, Stanley Wolf, Lattice Press, 1990; and
Flash memories: the best of two worlds, IEEE Spectrum, December 1989.